Pn-junction phase modulator in a large silicon waveguide platform

ABSTRACT

A modulator. In some embodiments, the modulator includes a portion of an optical waveguide, the waveguide including a rib extending upwards from a surrounding slab. The rib may have a first sidewall, and a second sidewall parallel to the first sidewall. The rib may include a first region of a first conductivity type, and a second region of a second conductivity type different from the first conductivity type. The second region may have a first portion parallel to and extending to the first sidewall, and a second portion parallel to the second sidewall. The first region may extend between the first portion of the second region and the second portion of the second region.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and the benefit of U.S.Provisional Application No. 62/750,226, filed Oct. 24, 2018, entitled“PN-JUNCTION PHASE MODULATOR IN A LARGE SILICON WAVEGUIDE PLATFORM”, theentire content of which is incorporated herein by reference.

FIELD

One or more aspects of embodiments according to the present disclosurerelate to modulators, and more particularly to a PN-junction phasemodulator in a large silicon waveguide platform.

BACKGROUND

A large silicon waveguide may have significant advantages over, e.g., asubmicron silicon waveguide in terms of fabrication insensitivity andpolarization independence. In a free-carrier-based phase modulator,however, the bandwidth of the device may scale approximately as theinverse of the volume of the active section of the waveguide. Because ofthis, achieving GHz-rate phase modulation may be challenging in a largewaveguide platform.

An important limitation on the speed of the modulator may be due to itscapacitance. It may therefore be advantageous for the modulator toachieve a given phase shift in a short device length. Moreover, thefabrication of a pn-junction in a large silicon waveguide platform maybe challenging due to the limited ion implantation depth of ionimplantation processes.

Thus, there is a need for an improved phase modulator in a large siliconwaveguide platform.

SUMMARY

According to some embodiments of the present invention, there isprovided a modulator, including: a portion of an optical waveguide, theoptical waveguide including a rib extending upwards from a surroundingslab; the rib having a first sidewall, and a second sidewall parallel tothe first sidewall; the rib including a first region of a firstconductivity type, a second region of a second conductivity typedifferent from the first conductivity type, and a third region of thefirst conductivity type; the second region having: a first portionparallel to and extending to the first sidewall, and a second portionparallel to the second sidewall, the third region being parallel to andextending to the second sidewall; the first region extending between thefirst portion of the second region and the second portion of the secondregion; and the second portion of the second region being between thefirst region and the third region.

In some embodiments, the ratio of the height of the rib to the width ofthe rib is greater than 2 and less than 6.

In some embodiments, the first region has a keyhole shape including arounded upper portion having a width exceeding, by at least 5%, a widthof a narrower lower portion.

In some embodiments, an interface between the first region and thesecond region includes two parallel vertical portions, the modulatorbeing configured to impose a phase shift on light propagating throughit, in response to a reverse-bias voltage applied being applied acrossthe first region and the second region, at least 90% of the phase shiftbeing due to interaction of the light with the two parallel verticalportions.

In some embodiments, the modulator further includes: a first metalcontact, and a second metal contact, the first metal contact beingconnected to the first region through a conductive path traversing, in adirection from the first metal contact to the first region: first, aheavily doped region of the first conductivity type, and second, a dopedregion of the first conductivity type, with a doping level between thatof the heavily doped region of the first conductivity type and that ofthe first region; and the second metal contact being connected to thesecond region through a conductive path traversing, in a direction fromthe second metal contact to the second region: first, a heavily dopedregion of the second conductivity type, and second, a doped region ofthe second conductivity type, with a doping level between that of theheavily doped region of the second conductivity type and that of thesecond region.

In some embodiments, the first conductivity type is p-type and thesecond conductivity type is n-type.

In some embodiments, the rib has a height of at least 1.8 microns andless than 4 microns and a width of at least 0.5 microns and less than1.5 microns.

In some embodiments, the rib is composed of crystalline silicon or ofcrystalline silicon germanium.

According to some embodiments of the present invention, there isprovided a method for fabricating a modulator on a semiconductor wafer,the method including: performing a first ion implantation operation on arib, the rib extending upwards from an upper surface of thesemiconductor wafer and having a first sidewall, and a second sidewallparallel to the first sidewall; and performing a second ion implantationoperation on the rib, wherein: the implantation angle of the first ionimplantation operation is greater than 45 degrees, the implantationangle of the second ion implantation operation is greater than 45degrees, and the azimuth of the direction of the first ion implantationoperation differs from the azimuth of the direction of the second ionimplantation operation by between 150 and 210 degrees.

In some embodiments, the method further includes: performing a third ionimplantation operation on the rib, and performing a fourth ionimplantation operation on the rib, wherein: the implantation angle ofthe third ion implantation operation is greater than 45 degrees, theimplantation angle of the fourth ion implantation operation is greaterthan 45 degrees, and the azimuth of the direction of the first ionimplantation operation differs from the azimuth of the direction of thesecond ion implantation operation by between 150 and 210 degrees.

In some embodiments: both the first and second ion implantationoperations are performed before the third ion implantation operation andbefore the fourth ion implantation operation, the first and second ionimplantation operations implant dopants of a first conductivity type,and the third and fourth ion implantation operations implant dopants ofa second conductivity type, different from the first conductivity type.

In some embodiments, the first conductivity type is p-type and thesecond conductivity type is n-type.

In some embodiments, the method further includes forming a barrier,before performing the third ion implantation operation and beforeperforming the fourth ion implantation operation, the barrier beingconfigured to at least partially shade at least a lower portion of asidewall of the rib from ions during the third ion implantationoperation.

In some embodiments, the fourth ion implantation operation is performedbefore the third ion implantation operation.

In some embodiments, the barrier is a layer of photoresist, separatedfrom the rib by a gap.

In some embodiments, the thickness of the layer of photoresist isgreater than 0.6 times the height of the rib, and less than 2.5 timesthe height of the rib.

In some embodiments, the width of the gap is greater than 0.4 times thethickness of the layer of photoresist and less than 1.2 times thethickness of the layer of photoresist.

In some embodiments, the method further includes performing a fifth ionimplantation operation on the rib, wherein: the first, second, and fifthion implantation operations implant dopants of a first conductivitytype, and the fifth ion implantation operation is performed at a lowerimplantation energy than the first ion implantation operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present disclosure willbe appreciated and understood with reference to the specification,claims, and appended drawings wherein:

FIG. 1A is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 1B is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 1C is a schematic cross-sectional top view of a modulator,according to an embodiment of the present disclosure;

FIG. 1D is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 1E is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 1F is a graph of doping profiles of modulators, according to someembodiments of the present disclosure;

FIG. 1G is a graph of a doping profiles of a modulator, according to anembodiment of the present disclosure;

FIG. 2 is a table of modulator parameters, according to some embodimentsof the present disclosure;

FIG. 3A is a schematic cross-sectional view of an intermediate productin a process for fabricating a modulator, according to an embodiment ofthe present disclosure;

FIG. 3B is a schematic cross-sectional view of an intermediate productin a process for fabricating a modulator, according to an embodiment ofthe present disclosure;

FIG. 3C is a schematic cross-sectional view of an intermediate productin a process for fabricating a modulator, according to an embodiment ofthe present disclosure;

FIG. 3D is a schematic cross-sectional view of an intermediate productin a process for fabricating a modulator, according to an embodiment ofthe present disclosure;

FIG. 4A is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 4B is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 4C is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 4D is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 4E is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 4F is a schematic cross-sectional view of a modulator, according toan embodiment of the present disclosure;

FIG. 5A is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 5B is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 5C is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 5D is cross-sectional density diagram, according to an embodimentof the present disclosure;

FIG. 6A is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure;

FIG. 6B is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure;

FIG. 6C is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure;

FIG. 6D is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure;

FIG. 7A is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure; and

FIG. 7B is a graph of simulated modulator characteristics, according toan embodiment of the present disclosure.

In the drawings, each of the density diagrams and graphs is drawn toscale, for a respective embodiment. The remaining drawings are not drawnto scale.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of apn-junction phase modulator in a large silicon waveguide platformprovided in accordance with the present disclosure and is not intendedto represent the only forms in which the present disclosure may beconstructed or utilized. The description sets forth the features of thepresent disclosure in connection with the illustrated embodiments. It isto be understood, however, that the same or equivalent functions andstructures may be accomplished by different embodiments that are alsointended to be encompassed within the scope of the disclosure. Asdenoted elsewhere herein, like element numbers are intended to indicatelike elements or features.

FIG. 1A shows a pn-junction modulator, in some embodiments. In FIG. 1A,the pn-junction has a keyhole shape, where the majority of the junctionconsists of the two vertical p-n interfaces 110 parallel to thewaveguide sidewall. The two vertical p-n interfaces 110 are connected atthe top by a round p-n interface 115. The rounded upper portion (definedby the round p-n interface 115) of the keyhole shape may have a widthexceeding (e.g., by at least 5%) the width of the narrower, straight,lower portion defined by the two vertical p-n interfaces 110. The shapeof the round part of the pn-junction at the waveguide top (i.e., of theround p-n interface 115) may vary with the waveguide width, and maybecome more semicircular at narrow waveguide widths (e.g., for a widthof 600 nm, for a 3 micron-tall (3 um-tall) waveguide, or for a waveguidehaving a height between 1 micron and 3 microns)(see FIG. 1D, forexample). Dimensions of some of the features are shown in FIG. 1A, forone embodiment.

The pn-junction modulator may be fabricated using a silicon on insulator(SOI) wafer, having a silicon substrate, or “silicon handle” 120, aburied oxide (BOX) layer 125, and an upper silicon layer, on the BOXlayer 125. During fabrication of the pn-junction modulator, variousetching steps and other processing steps (e.g., deposition steps and ionimplantation steps, as discussed in further detail below) may beperformed that may alter the shape and/or the composition of the uppersilicon layer. The waveguide may include a rib 130 of crystallinesilicon extending upwards from a surrounding slab 132. A layer ofsilicon dioxide 135 may cover the rib 130. The crystalline silicon ofthe rib 130 may have an index of refraction, within the wavelength rangeof interest (e.g., in the range between 1500 and 1600 nm), that issignificantly greater than that of the layer of silicon dioxide 135 (andthan that of the air, or other material, that may be present on theother side of the layer of silicon dioxide 135), so that the eigenmodesof the waveguide may be nearly fully confined to the rib 130, with onlya small fraction of the optical power being in evanescent waves outsideof the rib 130 or in the slab 132. The rib 130 may include a centraldoped region 140 of a first conductivity type (e.g., a p-doped region)and a surrounding region 145 of a second conductivity type (e.g., ann-doped region), the interface surface between which forms the verticalp-n interfaces 110 and the round p-n interface 115.

The silicon slab 132 adjacent to the rib 130 may have heavily dopedregions in contact with the n- and p-doped regions of the pn-junctionmodulator. For example, a heavily n-doped region 150 may be in contact(either directly, or, as discussed below, through a region ofintermediate doping level) with the n-doped surrounding region 145, anda metal contact 155 (e.g., an aluminum contact) may in turn be incontact with the heavily n-doped region 150. This arrangement mayprovide a current path with low series and contact resistance betweenthe metal contact 155 and the n-doped surrounding region 145. Similarly,a heavily p-doped region 160 may be in contact (either directly, or, asdiscussed below, through a region of intermediate doping level) with thecentral p-doped region 140, and a metal contact 165 (e.g., an aluminumcontact) may in turn be in contact with the heavily p-doped region 160,providing a current path with low series and contact resistance betweenthe metal contact 165 and the central p-doped region 140. Intermediatedoping level regions (an N+ region 152 and a P+ region 162) may beformed (i) between the n-doped surrounding region 145 and the heavilyn-doped region 150, and (ii) between the central p-doped region 140 andthe heavily p-doped region 160, respectively. The presence of suchintermediate doping level regions may reduce the insertion loss of themodulator while maintaining the modulator performance (e.g., the phaseefficiency and speed of the modulator). FIG. 1B shows an example of adoping profile for a structure like that of FIG. 1A. FIG. 1C shows aschematic top view of the phase modulator, showing the implantedregions, the metal contacts 155,165 and the waveguide rib 130.

FIG. 1D shows a pn-junction modulator, in some embodiments. Thepn-junction modulator of FIG. 1D has a narrower waveguide than thepn-junction modulator of FIG. 1A, and the round p-n interface 115 isaccordingly more semicircular than the round p-n interface 115 of thepn-junction modulator of FIG. 1A. FIG. 1E shows an example of a dopingprofile for a structure like that of FIG. 1D. In some embodiments, theround shape of the round p-n interface 115 is a result of the rib'sdoping profile being formed using ion implantation doping with largetilt angles (i.e., large declination angles), and without the use ofvertical, or zero-degree tilt ion implantation, as discussed in furtherdetail below.

A pn junction phase modulator such as those of FIGS. 1A and 1D, and ofother similar embodiments, may be fabricated in a large siliconwaveguide platform in which the waveguide thickness is larger than 1 um(the thickness being the dimension in the direction perpendicular to theplane of the substrate). The pn-junction phase modulator may be operatedwith a reverse bias and may have an efficiency VπL<1.2 V cm. From theleft side of the waveguide to the right side, the doping profile is‘n-p-n’. As the reverse bias voltage increases, the free holes in thep-region are depleted. The depletion may be significantly more efficientwhen two vertical p-n interfaces 110 are present than in embodimentswith only one such vertical interface. Vertical n- and p-doped sectionsof the waveguide may be fabricated using large implant tilt angles, asdiscussed in further detail below.

FIG. 1F is a graph of the doping profile of a phase modulator with awaveguide width of 800 nm, taken (i) along the line A-A′ of FIG. 1A (ina first curve 180) and (ii) along the line A-A′ of FIG. 4C (in a secondcurve 185). FIG. 1G is a graph of a doping profile of the slab region ofthe phase modulator of FIG. 1A, taken along the line B-B′. As mentionedabove, intermediate doping level regions (regions 152, 162 (FIG. 1A)with N+ and P+ doping levels) may be present.

In some embodiments, the aspect ratio of the waveguide, which is theratio of the waveguide height to its width, is larger than 3. In somecases, it is less than 6. The aspect ratio may affect the performance ofthe modulator. For example, the table of FIG. 2 shows dimensions of someembodiments, with waveguide heights (i.e., thicknesses) ranging from 1.0microns to 3.0 microns, and widths ranging from 0.5 microns to 1.0microns. Ranges of aspect ratios are shown in the third column of thetable of FIG. 2. A figure of merit for a phase modulator may be theratio of (i) the product of the length of the modulator and the voltagethat will produce a π phase shift to (ii) the 3 dB bandwidth of themodulator. In the table of FIG. 2, the fourth, fifth and sixth columnshow the waveguide width for which the figure of merit may have highervalues, the corresponding aspect ratio, and the corresponding phaseshift contribution from the vertical p-n interfaces 110, respectively.In some embodiments, a modulator may have a length between 2.0 mm and3.0 mm and it may achieve a phase shift of π for a modulating voltage ofbetween 4.0 V and 6.0 V. The short device length may enable high speedphase modulation in the GHz bandwidth regime.

FIGS. 3A-3D show process steps for fabricating a modulator, in someembodiments. FIG. 3A shows an ion implantation step in which the heavilyp-doped region 160 is formed by implantation of ions, e.g., boron ionsas shown. The rib 130 may have, on its top surface, a thicker layer ofsilicon dioxide that functions as a hard mask 320 and blocks the boronions, so that during this step they are not implanted in the rib 130.Photoresist 310 may shield all of the device except for (i) the portionof the slab 132 where the heavily p-doped region 160 is to be formed,and (ii) a portion of the rib 130, as shown (only a portion of the rib130 is shielded, the rib itself being shielded by the hard mask 320, sothat imprecision in the formation of the photoresist 310 will not affectthe edge of the heavily p-doped region 160 nearest the rib 130). Thetilt angle, which in FIG. 3A refers to an elevation angle measured fromthe nadir, may be between 0 degrees and 7 degrees, for example, andaccordingly the implantation angle may be between 90 degrees and 83degrees. The P+ intermediate doping level region 162 may be formed in asimilar ion implantation step (performed before or after the ionimplantation step used to form the heavily p-doped region 160), with atilt angle of about 1.5 degrees, and a lower implantation dose than thatused to form the heavily p-doped region 160. If a P+ intermediate dopinglevel region 162 is to be formed, a tilt angle of greater than 0 degreesmay be used for the high dose ion implantation step (used to form theheavily p-doped region 160), and, during the high dose ion implantationstep, the intermediate doping level region 162 may be shielded by thehard mask 320. As used herein, the “implantation angle” is the angle ofdeclination (or angle of depression) (measured from the horizontal), ofthe direction of the ions in an ion implantation operation. The azimuthof the direction of the ions may be such that the top of the rib 130shields the sidewall of the rib that is not protected by photoresist310.

FIG. 3B shows an ion implantation step in which the heavily n-dopedregion 150 is formed by implantation of ions, e.g., phosphorous ions(“Phos”), as shown, or arsenic ions. As in the operation illustrated inFIG. 3A, the rib 130 may have a silicon dioxide hard mask 320 on its topsurface, and photoresist 310 may shield all of the device except for (i)the portion of the slab 132 where the heavily n-doped region 150 is tobe formed, and (ii) a portion of the rib 130, as shown. The tilt angle,which in FIG. 3B also refers to an elevation angle measured from thenadir, may be between 0 degrees and 7 degrees, for example. The azimuthof the direction of the ions may be such that the top of the rib 130shields the sidewall of the rib that is not protected by photoresist310. In a step analogous to the step described above for forming the P+intermediate doping level region 162, an N+ intermediate doping levelregion 152 may be formed by (i) shielding the corresponding region fromthe high dose of n-dopant by the hard mask 320 (and using a tilt angleof greater than 0 degrees for the high dose ion implantation step) and(ii) performing a separate, lower dose implantation step with a lowertilt angle (e.g., about 1.5 degrees).

FIG. 3C shows an ion implantation step in which the p-dopant ions (e.g.,boron ions) are implanted in the rib 130. The tilt angle, which in FIG.3C refers to an angle of declination (or an angle of depression)measured from the horizontal, may be between 45 degrees and 60 degrees,for example. Photoresist 310 may partially cover the heavily n-dopedregion 150. This may have the benefit of protecting the heavily n-dopedregion 150 from implantation by p-dopant ions, although the p-dopantdose may be sufficiently low that the doping concentration of theheavily n-doped region 150 is affected only slightly in any portion ofthe heavily n-doped region 150 that is not protected by photoresist 310(e.g., the N++ doping concentration may be about 2e20/cm{circumflex over( )}3 and the P doping concentration added in this step may be betweenabout 1e18/cm{circumflex over ( )}3 and about 2e18/cm{circumflex over( )}3, so that the net doping concentration in the heavily n-dopedregion 150 may be affected only slightly by whether the heavily n-dopedregion 150 is exposed to the implantation by p-dopant ions). In someembodiments photoresist is entirely absent from the heavily n-dopedregion 150 or from the partially fabricated modulator during this step(photoresist may however be present on other parts of the chip toprotect other structures). The separation w_p between the edge of thephotoresist and the rib 130 may be chosen such thatw_p=t_PR*tan(θ_(p))+margin_p, where t_PR is the thickness of thephotoresist, θ_(p) is the implantation angle, and margin_p is anadjustable parameter that may be set (e.g., so that margin_p>1 um) toassure that the whole rib section is implanted with boron. After thestep of FIG. 3C is completed, the hard mask 320 (and the oxide layer onthe sidewalls of the waveguide and the surrounding slab 132) may beremoved using a wet etch, and replaced with a new oxide layer that maybe thinner than the hard mask 320. For example, the new oxide layer maybe 50 nm thick.

FIG. 3D shows an ion implantation step in which the n-dopant ions (e.g.,phosphorus or arsenic ions) are implanted in the rib 130, to form thesurrounding (n-doped) region 145 and to form (as the central remainderof the rib, that is not converted from p-type to n-type by this ionimplantation step) the central p-doped region 140. In a manner analogousto that of the step illustrated in FIG. 3C, photoresist 310 maypartially cover the heavily p-doped region 160. As in the case of thestep illustrated in FIG. 3C, the presence of photoresist 310 may beoptional and in some embodiments some or all of the photoresist 310illustrated in FIG. 3D is absent. The central doped region 140 mayinclude a shelf 315 on the side of the heavily p-doped region 160; theshelf may have a height D above the upper surface of the heavily p-dopedregion 160 (for negative values of D, the height of the shelf 315 may belower than that of the upper surface of the heavily p-doped region 160).In some embodiments, the doping operations may be performed in adifferent order than that described in some examples herein; forexample, the operations of FIG. 3D may be performed before those of FIG.3C.

The separation w_n between the edge of the photoresist and the rib 130may be lithographically controlled for target junction shape, accordingto whether or not a self-aligned process is used. In a first embodiment,a self-aligned process is used, and w_n may be chosen according tow_n=t_PR*tan(θ_(n))+margin_n, where margin_n is an adjustable parameterthat may be set to be larger than 0, e.g., larger than 0.5 um, to ensurethat the self-aligned junction has an acceptable shape even with processvariation of t_PR and w_n. In this first embodiment, D may be between−100 nm and 0. In a second embodiment, w_n may be chosen according tow_n=(t_PR-D)*tan(θ_(n)) where D is between 0 and (t-t_slab), where t isthe height of the rib 130 and t_slab is the thickness of the slab 132.

FIGS. 4A-4E show modulator structures, according to some embodiments.FIG. 4A shows how the shape of the wrap-around junction (i.e., thejunction, which includes the two vertical p-n interfaces 110 and theround p-n interface 115) may be affected by photolithography, using aself-aligned fabrication process. In the self-aligned case, themodulator structure, and, in particular the junction shape may berelatively unaffected by incremental changes in the photoresistthickness or mask alignment. For example, as mentioned above, if D ischosen to be less than 0 (in FIG. 3D), then the alignment of thephotoresist in FIGS. 3A-3D has little effect on the modulator structure(the presence or absence of photoresist on the heavily doped regions150, 160 having little effect on their net doping concentrations). Bycontrast, the process of FIG. 3D is not self-aligned if D is chosen tobe greater than 0 (as shown in FIG. 4B), because in this case variationsin the alignment of the photoresist 310 may affect the modulatorstructure (by affecting D).

FIGS. 4C-4F show embodiments in which an additional, or “secondary”, Pregion 170 (FIGS. 4C and 4E), or an intrinsic region 175 (FIGS. 4D and4F) may be made at a waveguide sidewall (and, in some embodiments (FIGS.4E and 4F), at the top of the waveguide, forming, for example, a regionhaving an inverted-L shape) by a counter-doping method. In such anembodiment, the junction profile may become N/P/N/P or N/P/N/i where Pand i refer to the secondary P region 170 and the intrinsic region 175,respectively. A modulator according to FIGS. 4C and 4D, with a secondaryP region or an intrinsic region (labeled “i”) may have smallercapacitance, larger bandwidth, and lower loss than a device without sucha region. The embodiments of FIGS. 4C and 4D may be fabricated byperforming an additional, “secondary” ion implantation operation withp-dopant ions, at the step illustrated in FIG. 3A, with an implantationangle and implantation energy selected to implant p-dopant ions justbelow the surface of the right sidewall of the rib 130. This secondaryion implantation operation may be performed, for example, with animplantation energy corresponding to an accelerating voltage of 90 kVand a tilt angle of 10 degrees from the vertical. The azimuth of thedirection of the ions may be such that the ions impinge on and penetratethe right sidewall of the rib 130. The subsequent ion implantationoperation of FIG. 3C may then be performed with an implantation energycorresponding to an accelerating voltage of 150 kV. The embodiments ofFIGS. 4E and 4F may be fabricated by instead performing the secondaryion implantation operation, with p-dopant ions, before the stepillustrated in FIG. 3D, i.e., after the hard mask 320 has been removedand replaced with the thinner oxide layer. The absence of the hard mask320 during the secondary ion implantation operation may then allow theions to be implanted at the top surface of the waveguide rib 130, inaddition to the sidewall. As a result, the secondary P region 170, orthe intrinsic region 175, may have an inverted-L shape, as shown forexample in FIGS. 4E and 4F. The intrinsic regions of FIGS. 4D and 4F maybe formed in a structure such as that of FIG. 4C or 4E, respectively, byapplication of a suitable reverse bias. In some embodiments, theintrinsic regions of FIGS. 4D and 4F may be formed by suitable selectionof doping doses during the counter-doping operation, such that the netcarrier level after doping and counter-doping is zero or nearly zero inthe intrinsic regions.

FIGS. 5A and 5B show the simulated doping concentration, and thesimulated hole concentration, respectively, of an embodiment (e.g., thedevice of FIG. 4C) having a self-aligned junction with a secondary Pregion 170 in the waveguide, at 0 V bias. From FIG. 5B it may be seenthat the secondary P region 170 on the right side of the waveguide rib130 has net P-doping. FIG. 5C shows the simulated hole concentration ofthe device in FIG. 4C at 1.2 V reverse bias, showing that holes in theright side of the waveguide are depleted, effectively creating the thinintrinsic region. FIG. 5D shows the simulated hole concentration of thedevice in FIG. 4C at 5.0 V reverse bias. It can be seen from FIG. 5Dthat at this bias, holes at the center of the waveguide are mostlydepleted.

FIGS. 6A-6D show graphs of simulated device performance versus waveguidewidth, for a device such as that of FIG. 4C, for two cases, (i) without(n-p-n) and (ii) with (n-p-n-p) secondary P doping (forming a secondaryP region 170). In the example to which FIGS. 6A-6D apply, the length ofthe simulated device is 2.5 mm. FIG. 6A shows Vmax as a function ofwaveguide width. Vmax is defined such that 0.75 pi phase shift isachieved between V=1.2 V and Vmax. The difference in Vmax is less than1% between the two cases. FIG. 6B shows the average 3 dB bandwidth as afunction of waveguide width when the phase modulator is modulatedbetween V=1.2 V and Vmax. The 3 dB bandwidth takes into account anoutput impedance of 9 ohms in the driver connected to the phasemodulator.

FIG. 6C shows the average attenuation as a function of waveguide widthwhen the phase modulator is modulated between V=1.2 V and Vmax. It maybe seen from FIG. 6C that when secondary P doping is used, a reductionin the average attenuation of the phase modulator may be achieved, dueto the decreased N concentration in the waveguide. For example, at awaveguide width of 675 nm, the presence of a secondary P region 170lowers the average attenuation by 0.7 dB. FIG. 6D shows a figure ofmerit as a function of waveguide width when the phase modulator ismodulated between V=1.2 V and Vmax. For FIG. 6D, the figure of merit isdefined as the average 3 dB bandwidth divided by (Vmax−1.2 V).

In some embodiments, a pn junction phase modulator may be nearlypolarization-independent in the sense that it has very smallpolarization-dependent phase change and absorption. FIG. 7A shows thesimulated phase shift as a function of reverse bias voltage for thefundamental TE and TM modes of a modulator according to the embodimentof FIG. 4C, with a length of 2.5 mm, and a waveguide width of 0.75microns. In this example, the device has secondary P doping (forming asecondary P region 170). The simulation used to generate FIG. 7A showsthat Vπ=4.24 V for the TE mode, and Vπ=4.36 V for the TM mode; thedifference between these two values is 2.8%. For the fundamental TEmode, VπL=1.06 V cm. FIG. 7B show attenuation as a function of reversebias voltage for the fundamental TE and TM modes of a modulatoraccording to the embodiment of FIG. 4C, with a length of 2.5 mm, and awaveguide width of 0.75 microns. The attenuation may be attributed tofree carrier absorption.

Any numerical range recited herein is intended to include all sub-rangesof the same numerical precision subsumed within the recited range. Forexample, a range of “1.0 to 10.0” or “between 1.0 and 10.0” is intendedto include all subranges between (and including) the recited minimumvalue of 1.0 and the recited maximum value of 10.0, that is, having aminimum value equal to or greater than 1.0 and a maximum value equal toor less than 10.0, such as, for example, 2.4 to 7.6. Any maximumnumerical limitation recited herein is intended to include all lowernumerical limitations subsumed therein and any minimum numericallimitation recited in this specification is intended to include allhigher numerical limitations subsumed therein.

Although exemplary embodiments of a pn-junction phase modulator in alarge silicon waveguide platform have been specifically described andillustrated herein, many modifications and variations will be apparentto those skilled in the art. Accordingly, it is to be understood that apn-junction phase modulator in a large silicon waveguide platformconstructed according to principles of this disclosure may be embodiedother than as specifically described herein. The invention is alsodefined in the following claims, and equivalents thereof.

What is claimed is:
 1. A modulator, comprising: a portion of an opticalwaveguide, the optical waveguide comprising a rib extending upwards froma surrounding slab; the rib having a first sidewall, and a secondsidewall parallel to the first sidewall; the rib including a firstregion of a first conductivity type, a second region of a secondconductivity type different from the first conductivity type, and athird region of the first conductivity type; the second region having: afirst portion parallel to and extending to the first sidewall, and asecond portion parallel to the second sidewall, the third region beingparallel to and extending to the second sidewall; the first regionextending between the first portion of the second region and the secondportion of the second region; and the second portion of the secondregion being between the first region and the third region.
 2. Themodulator of claim 1, wherein the ratio of the height of the rib to thewidth of the rib is greater than 2 and less than
 6. 3. The modulator ofclaim 1, wherein the first region has a keyhole shape including arounded upper portion having a width exceeding, by at least 5%, a widthof a narrower lower portion.
 4. The modulator of claim 1, wherein aninterface between the first region and the second region includes twoparallel vertical portions, the modulator being configured to impose aphase shift on light propagating through it, in response to areverse-bias voltage applied being applied across the first region andthe second region, at least 90% of the phase shift being due tointeraction of the light with the two parallel vertical portions.
 5. Themodulator of claim 1, further comprising: a first metal contact, and asecond metal contact, the first metal contact being connected to thefirst region through a conductive path traversing, in a direction fromthe first metal contact to the first region: first, a heavily dopedregion of the first conductivity type, and second, a doped region of thefirst conductivity type, with a doping level between that of the heavilydoped region of the first conductivity type and that of the firstregion; and the second metal contact being connected to the secondregion through a conductive path traversing, in a direction from thesecond metal contact to the second region: first, a heavily doped regionof the second conductivity type, and second, a doped region of thesecond conductivity type, with a doping level between that of theheavily doped region of the second conductivity type and that of thesecond region.
 6. The modulator of claim 1, wherein the firstconductivity type is p-type and the second conductivity type is n-type.7. The modulator of claim 1, wherein the rib has a height of at least1.8 microns and less than 4 microns and a width of at least 0.5 micronsand less than 1.5 microns.
 8. The modulator of claim 1, wherein the ribis composed of crystalline silicon or of crystalline silicon germanium.9. A method for fabricating a modulator on a semiconductor wafer, themethod comprising: performing a first ion implantation operation on arib, the rib extending upwards from an upper surface of thesemiconductor wafer and having a first sidewall, and a second sidewallparallel to the first sidewall; and performing a second ion implantationoperation on the rib, wherein: the implantation angle of the first ionimplantation operation is greater than 45 degrees, the implantationangle of the second ion implantation operation is greater than 45degrees, and the azimuth of the direction of the first ion implantationoperation differs from the azimuth of the direction of the second ionimplantation operation by between 150 and 210 degrees.
 10. The method ofclaim 9, further comprising: performing a third ion implantationoperation on the rib, and performing a fourth ion implantation operationon the rib, wherein: the implantation angle of the third ionimplantation operation is greater than 45 degrees, the implantationangle of the fourth ion implantation operation is greater than 45degrees, and the azimuth of the direction of the first ion implantationoperation differs from the azimuth of the direction of the second ionimplantation operation by between 150 and 210 degrees.
 11. The method ofclaim 10, wherein: both the first and second ion implantation operationsare performed before the third ion implantation operation and before thefourth ion implantation operation, the first and second ion implantationoperations implant dopants of a first conductivity type, and the thirdand fourth ion implantation operations implant dopants of a secondconductivity type, different from the first conductivity type.
 12. Themethod of claim 11, wherein the first conductivity type is p-type andthe second conductivity type is n-type.
 13. The method of claim 11,further comprising forming a barrier, before performing the third ionimplantation operation and before performing the fourth ion implantationoperation, the barrier being configured to at least partially shade atleast a lower portion of a sidewall of the rib from ions during thethird ion implantation operation.
 14. The method of claim 13, whereinthe fourth ion implantation operation is performed before the third ionimplantation operation.
 15. The method of claim 13, wherein the barrieris a layer of photoresist, separated from the rib by a gap.
 16. Themethod of claim 15, wherein the thickness of the layer of photoresist isgreater than 0.6 times the height of the rib, and less than 2.5 timesthe height of the rib.
 17. The method of claim 15, wherein the width ofthe gap is greater than 0.4 times the thickness of the layer ofphotoresist and less than 1.2 times the thickness of the layer ofphotoresist.
 18. The method of claim 10, further comprising performing afifth ion implantation operation on the rib, wherein: the first, second,and fifth ion implantation operations implant dopants of a firstconductivity type, and the fifth ion implantation operation is performedat a lower implantation energy than the first ion implantationoperation.